Composite materials are generally considered engineering materials made from two or more components. One component is often a fiber or porous solid phase, generally called the strengthener, which gives the material its tensile strength, while another component is often a resin or liquid phase, generally called the matrix, which binds the strengthener together and renders the composite material generally either stiff and rigid or deformable as a whole.
Several processes used in the manufacture of composite parts consist of impregnating the strengthener with the matrix, which may be a polymer (a resin) or any material that is liquid at the injection temperature. Variants of this family of processes are now grouped, under the generic term LCM or Liquid Composite Molding. Although they are used principally for the manufacture of polymer matrix composite parts, LCM processes are also encountered in biomedical and electronics applications, for example when injecting a polymer to insulate microelectronic circuits. A molten metal may be injected instead of a polymer resin in order to manufacture metal matrix composite parts.
Polymer matrix composite manufacturing processes may be separated into several categories:
Contact Molding Process
This manual method generally uses a half-mold on which the dry strengthener is set in layers that are impregnated by hand.
Autoclave Process
The composite part is generally manufactured by hand from pre-impregnated strengtheners and then cured in an autoclave. This method is widely used in aeronautics, most notably in the military sector. However, its cost remains generally high.
RTM (Resin Transfer Molding) Process
The matrix in its liquid state is generally injected throughout a strengthener confined in a rigid mold. Liquid injection in the RTM process may be carried out at ambient temperature or at a higher temperature by heating either the injected liquid, the mold or both.
VARTM (Vacuum Assisted Resin Transfer Molding) Process
A vacuum is generated inside the mold in order to facilitate and accelerate matrix injection.
CRTM (Compression Resin Transfer Molding) Process
Also called injection-compression, the CRTM process generally consists of opening slightly the gap between the mold halves during injection to accelerate the liquid flow, after which the part is consolidated and sized to specification by lowering the press on the molds or one of the mold halves on the other.
VARI (Vacuum Assisted Resin Infusion) Processes
The strengthener is generally arranged beneath a plastic film or elastic membrane, creating a compartment which may be placed under vacuum. The liquid then infuses into the strengthener by gravity.
RTM Light Process
This variant combines the advantages of deforming one boundary of the mold, as in the VARI process, with an imposed injection pressure, as in the RTM process. The mold consists of one or two thin metal or composite shells, which may be deformed under the pressure of injection. A first vacuum usually ensures closure of the mold, while a second vacuum is generated inside the cavity to accelerate injection.
Other Variants
Numerous other variants of the LCM processes exist, which may be associated with one or another of the previously described main categories. For example, the VEC injection process uses reservoirs containing a non-compressible fluid to strengthen the walls of a mold that are not in contact with the part. Preferential flow channels may also be created by various means in an outer layer of the strengthener (SCRIMP process), in one of the mold walls or inside the cavity, in order to facilitate liquid infusion or injection.
The quality of parts manufactured by contact molding is generally lower on average than that of injected parts. Labor costs are considerable as well, since each layer of the strengthener must be precisely positioned in the cavity and the laminate impregnated by hand. During impregnation by the liquid, air bubbles are often entrapped inside the composite part. This constitutes the principal problem with contact molding and explains notably the large variations observed in part weight. A second disadvantage stems from the difficulty of ensuring constant part thickness and uniform fiber content, two critical parameters that govern the quality and mechanical properties of the composite part. Finally, another problem arises from increasingly strict government regulations concerning toxic gases or vapors generated during open mold manufacturing.
LCM processes based on the use of closed molds significantly eliminate most gaseous emissions during manufacturing. Conventional liquid injection molding is done using two rigid half-molds: the base generally designates the bottom portion of the mold, which remains immobile and the punch designates the top portion, which is raised in order to open the mold and free the part at the end of the manufacturing cycle. Between these two half-molds lies a cavity in which the strengthener is arranged and into which the injection occurs.
The RTM process and its variants VARTM and CRTM are generally appropriate for the manufacture of structural composite parts, but constant thickness remains difficult to achieve because of the non uniform shrinkage of the resin during the cure. It is not always easy to eliminate porosity completely in injected parts, even by creating vacuum in the cavity before injection. Finally, the biggest difficulty is associated with the injection time, which is generally too long for strengtheners with high fiber content (i.e., more than 50%).
Overall, average surface appearance, low geometrical precision of the parts and limits regarding fiber content and injection time all reduce the range of applications of RTM process and its derived processes such as heated RTM, VARTM and injection-compression (CRTM). One constraint peculiar to CRTM process should also be mentioned. In general, the punch closes along a vertical axis, which results in practically no compression of the strengthener in the vertical zones of the cavity, while maximal pressure is exerted in the horizontal zones. This problem, in addition to difficulties inherent to the complexity of the process and risks of air entrapment during the compression phase, significantly limit the applications of CRTM process. It should be noted that compression of one half-mold over the other may also be performed by zones, but this significantly complicates the manufacturing of the mold.
Recently, new vacuum impregnation processes (VARI) have been introduced, which present the advantage of not requiring a cover mold. In these so-called liquid aspiration infusion processes, the strengthener is still arranged in the mold cavities, but is then covered with an impermeable membrane sealed to the outer edges of the mold. The air inside the cavity formed between the membrane and the mold may then be evacuated using a vacuum pump. Atmospheric pressure then compacts the strengthener, while the liquid flows from an external source into the strengthener-filled cavity under vacuum. In this type of VARI process, liquid infusion into the strengthener is carried out under vacuum at low flow rate under the single effect of static pressure due to gravity. The inclusion of air bubbles is thus eliminated, solving one of the problems encountered in other injection variants. However, the effect of gravity generally introduces non uniformity into the impregnation of the strengthener for large parts. In spite of the apparent simplicity of VARI processes, problems persist because the flow of the viscous matrix, such as for example resin, is difficult through strengtheners of low permeability. The fiber contents and dimensional accuracy of parts infused by VARI process are generally lower than the levels that may be accommodated by RTM process. Since resistance to resin penetration increases with the distance to be crossed, portions of certain parts may remain dry while excesses of resin accumulate in other zones.
In order to resolve these problems, variants of these processes have been recently developed, which artificially increase local permeabilities in the strengthener and thus decrease filling time. These include resin-dispersing permeable felts on one surface of the strengthener, networks of tubes to distribute the liquid matrix flow throughout the cavity, preferential flow channels or grooves incorporated into the surface of the mold and so on. All of these methods pose particular problems. The use of felt leads to increased waste of material, which is incompatible with mass production. Networks of tubes and flow channels generate practical development difficulties, which can be overcome only at the expense of generally costly trial periods. Finally, infusion still remains excessively slow compared to injection. The pressure gradient driving liquid penetration is much greater in an injection process than the infusion gradient which cannot exceed ambient atmospheric pressure minus the residual pressure inside the cavity under vacuum.